The present invention relates to a method and apparatus for forming a thin organic film, especially a heterostructural film obtained by building up a plurality of types of thin organic films.
Recent developments in material techniques using organic molecules are significant. Demand has arisen for a new functional element to be realized, using organic molecules. In particular, elements utilizing an ultra thin film using organic molecules have been widely researched.
Examples of a conventional method of forming a thin organic film are a spin coating method, a vapor deposition method, and a Langmuir-Brodgett method. The Langmuir-Brodgett method in particular has attracted a great deal of attention, being the one and only thin film forming method which is currently capable of orientating organic molecules in units of .ANG. and building them up. In the following description, the Langmuir-Brodgett method will be referred to as the LB method, and a film formed by this method will be referred to as an LB film. Examples of an element to which the LB film may be applied are a MIS light-emitting element or a MIS transistor using the LB film as an insulating film; a photoelectric conversion element, an optical recording medium, and a variety of sensors using dye molecules; and a piezoelectric element having a polarized film structure. In addition, research has been carried out with the aim of utilizing the LB film as an ultra micro-patterning resist.
Using the LB method, amphiphatic monomolecules are developed on a water surface and then compressed to obtain a predetermined surface pressure, thereby forming a condensed film. Thereafter, a sample substrate is moved vertically across the monomolecular film, to coat a thin organic molecular film thereon. This technique is known as a vertical dipping method. On the other hand, a method in which a sample substrate is brought into contact parallel to a developed monomolecular film, in order for a thin organic molecular film to be coated on the substrate, is known as a horizontal adhering method. A structure of a stacked monomolecular film obtained by the above methods has three types, i.e., a Y-type, in which hydrophilic groups or hydrophobic groups are adjacent to each other, and X- and Z-types, in which the hydrophobic group is adjacent to the hydrophilic group. The X- and Z-types are changed in accordance with the hydrophilic or hydrophobic property of the substrate surface. In the Z-type, a hydrophilic group of a monomolecular film adhered first is in contact with the substrate surface, and monomolecular films are sequentially stacked such that a hydrophilic group of a second monomolecular film is adhered to a hydrophobic group of the first monomolecular film, and so on. In contrast, in the X-type, a hydrophobic group of a monomolecular film is in contact with the substrate surface, and monomolecular films are sequentially stacked such that a hydrophobic group of a second monomolecular film is adhered to a hydrophilic group of the first monomolecular film, and so on.
However, according to either of the above methods, only a stacked monomolecular film consisting of a single component can be obtained. As a result, its field of application is limited. For this reason, an increasing amount of research is being carried out with the aim of developing a method of forming an alternate built-up film (heterostructural film) consisting of different types of molecules. In order to stack a plurality of types of organic molecular films on top of one another, it is necessary to develop different organic molecular films in different regions of a water tank and dip a sample substrate sequentially in the different regions where the different monomolecular films are developed. In any of the conventional alternate built-up film-forming apparatuses, a single water tank is partitioned by fixed barriers, to enable development of a plurality of types of monomolecular films. In this case, lower portions of the respective organic molecular film developing regions partitioned by the fixed barriers communicate with each other, and water is commonly used in all the developing regions. However, use of such a conventional alternate built-up film formation apparatus poses the following problems:
First, in order to develop a monomolecular film, the water phases (e.g., pH, temperature, and ion concentration) of the respective developing regions must be set to optimal values, in accordance with the organic molecules. However, where a common water tank is used, different pH values or temperatures cannot be set in the developing regions of each of the different monomolecular films. As a result, the range of different molecules which can be developed is limited. Moreover, in order to transport a sample substrate between the developing regions of the different monomolecular films, the following arrangements have been proposed:
(a) An arrangement in which a fixed barrier is formed by use of a flexible material, and a gate is provided at the fixed barrier so that a sample substrate support rod can pass therethrough; and
(b) an arrangement in which a fixed barrier is used as a rotating shaft and the sample substrate is supported by and rotated thereabout.
However, in the case of arrangement (a), if the sample substrate is transported through the gate, developed molecules will mix with each other or the surface pressure of the molecular films varies. This makes it difficult to form a good heterostructural film. Also, in the above two arrangements, the number of types of alternate built-up film is limited to two, i.e., three or more types of monomolecular films cannot be stacked. Furthermore, in these conventional apparatuses, the sample substrate must be transported inside the water tank. Therefore, there is danger that the thin organic film adhered on the substrate may be removed therefrom by water resistance.
In addition, since both of the above film formation methods are based on use of the vertical dipping method, only the Y-type hetero built-up film (in which hydrophilic groups or hydrophobic groups are adjacent to each other) can be obtained. For this reason, the above methods cannot sufficiently cope with the formation of a variety of films including other types of films required to develop a functional element which positively utilizes an interaction between different molecules. As has been described above, formation of a good hetero built-up film consisting of a plurality of types of molecules according to the conventional methods poses a number of problems in practice, and its application range is significantly limited.